EDIT: I have fixed the weird oscillation problem but still have problems with the HP part of the filter not closing completely/cutting off all sound. On top of that i would like to have a higher q factor - what resistor should i change?
Thanks!

(old post):
Hi again. I have an old formant elektor modular that i bought. When i bought it it was about 70% done - but every module has small weird problems that need working out.
-It has 2 vco's, 1 12 db filter, 2 adsr's, 1 vca, 1 triple LFO and 2 doepfer modules ( the envelope follower and noise/random module).

I finally managed to get both osc. to be stable but right now i'm working on the 12 db filter. It has 3 modes: LP/HP/BP. LP/BP seems to work fine, but i cant seem to get the HP mode to cut off the sound completely - i always get a some hi freq sound coming through even though the filter freq is at the extreme setting - on top of that i get erratic behaviour from the Q section. setting the cutoff freq knob to 12 o clock and the q to max i get SEVERE oscillation, but only when the cutoff freq is at 12 o clock.... its a great sounding filter so i hope i get it fixed..
As stated:
I get hi/lo/bandpass with various stages of succes..
I soldered in trimpots for r23, r27 and r6 so i could adjust everything instead
of relying on resistor that had a non difined value. But for the life of me i
cant get everything to work right.
-The LP works fine and i can get it to close all the way down/open fully. The HP
i cant get to close completely. I always have a bit of HF coming through.
Bandpass seems to work fine.
-A general problem with all the filter types:
If i leave freq in the middle and turn the q to max and adjust the trimpots so i
get maximum q value, i get a nice resonance that sound great. Then if i turn the
freq up i all of the sudden get a HUGE jump in q volume and the filter starts
self oscillating, but only at the extreme high freq setting - if the q was
consistent all over the freq. spectrum i could deal with it, but it is not...
Any suggestions? It seems to me that it could be an issue with a cap somewhere,
but which one(s)? It would make sense to me that it could be C2 since it's close
to IC1 which controls all of the various filter types.

Help much appreciated!
Actually here is a screenshot:

Last edited by the19thbear on Wed Feb 06, 2013 11:37 pm; edited 1 time in total

hmm... seems like switching the IC 741's with newer ones did the trick... i still have problems with HP not closing completely..
And another thing. What resistor should i change in order to get a higher q factor?

he 19thbear, wait a moment, had a post up yesterday, but took it down because i got it all wrong, to change the hp issue try decreasing the ratio of both the 33k/100ohm/100ohm (being 1:330) by maybe changing r20/r24 to 22k or even 10k...

just an idea to try, it´s a state variable filter and that worked for me on a sallen key with the same problem...and also allows to drive the filter harder!
for increasing q you need more negative feedback so although i don´t know this filtertopolgy, try lowering/raising r18...

also check out this link for some fundamentally better explanation than mine

Hi, well since as I said I´m not veteran about the state variable, in fact I´m only a hack with a little experience from experimenting!

So it´s only guesswork: but the thing with OTAs is R20/R21 and R24/R25 are voltage dividers to get the signal very small, the chips are only capable of handling small voltages, R22/R26 are there for symmetry of +/- inputs.
But they are also part of the gain equation of the OTA, which influences the filter frequency and the output will be at unity gain-

With the given values the signal is divided 100:33000 aka 1:330, now when I had a similar problem like you *but with a different filter topology* I studied datasheets and stuff and found out that the chip needed more gain, the problem got solved by raising the ratio of the divider to 1:100, but you even see like 1:33 aka 33k-1k-1k at the input, which is already overdrive territory, i think, but sounds good. Try!

You could also change the filter capacitor for different frequency, but that is like with short blanket you pull it up your feet are uncovered

If this chinese to you please tell! Since I´m also interested in this kind of filter I will do some reading and also we will sort out that that annoying resonance thing, btw did you try change R15?

Ok. So i solderen in another value for R24. Should be 39k but now is 10k and i can get the HP to close completely now! Thanks alot!
Ill have to continue with the q factor later. Tried R15 but got really weird behaviour! Self osc yes, but really low original signal and it just sounded trashed.. Any other ideas?
Thanks again!

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